Wish I knew the answer to that too !!, my puppy does the same exact thing, the entire time we are outside he is on a constant hunt to see what he can get into his mouth. Leaves are a big favorite. I think that as they get older it will stop, I figure right now they are just investigating everything.
Puppies explore with their mouths. They will pick up and try anything and everything to see if it tastes good. This is when you use the command leave it! Make sure they aren't picking up things like rocks because they can cause blockages.
Misty does the same thing. She's PRETTY good with the "leave it" command -- as long as I catch her in the act of "considering" grabbing something. Once she has it, it's a crap-shoot as to whether or not she'll drop it -- probably depending upon how good it tastes! :
Today, it was a huge wad of bubble gum. She went into the weeds sniffing, and came out chewing something vigorously. I got close enough to her to see a wad of gum in her mouth, and she was chewing happily! I tried to get it, but she went into butt-tucking mode, and for 2-3 minutes would NOT let me catch her. She ran around and around me in circles in full butt-tuck mode. When I finally did catch her, the gum was gone -- either she swallowed it, or dropped it while butt-tucking! : I know there is some sweetener (xylitol?) in some types of gum that can be bad for dogs, so I really wanted to get it from her. :-\ I watched her for awhile, though, and she seemed fine, so, I guess, no damage done. : It was just hilarious to watch a butt-tucking lab chewing away on a wad of bubble gum!!